In recent years, a drive recorder provided with a data storage is installed in automobiles to continuously record images captured by an image sensor. The drive recorder records video information around the automobile. The video information recorded on the data storage is reproduced afterward for use in analysis of the cause of a traffic accident and the like. A moving image recorded on the drive recorder is generally reproduced at an output frequency of 29.97 Hz adhering to the NTSC standard on a monitor or the like. For this reason, the image sensor captures one frame image at a frame frequency that is synchronized with the output frequency of the moving image.
On the other hand, in terms of energy saving, improvement in visibility and the like, LEDs (light emitting diodes) are widely used in traffic light machines. The LEDs used in the traffic light machine flash at a frequency twice as high as the frequency of a utility alternating current power supply of an installation area. For example, the frequency of the utility alternating current power supply is 60 Hz in western Japan, so that the LEDs of the traffic light machine installed in the western Japan flash at a frequency of 120 Hz.
Here, if the frame images are captured at a frame frequency of 29.97 Hz in synchronization with the output frequency 29.97 Hz for one frame image of the imaging device, as shown in FIG. 4, an imaging period is approximately a four times multiple of the flashing period of the LEDs and sometimes coincides with the turn-off timing of the LEDs for a certain period of time. In this case, therefore, the drive recorder cannot record which color of the traffic light machine is turned on for the certain period of time.
Thus, according to an imaging device of Patent Literature 1, a first field image is captured every field, and a second field image is captured after time of a one-fourth period of the frequency of the utility alternating current power supply has elapsed since the first field image is captured. By adding the captured first and second field images, one frame image is produced. The frame images are sequentially produced, and outputted as a moving image in the NTSC Format.
Also, in an imaging device of Patent Literature 2, the image sensor is exposed at a sufficiently lower frequency than the frequency of the utility alternating current power supply, i.e. for longer time than turn-off time in the flashing period of the LEDs, to capture one frame image. According to this conventional imaging device, the imaging period necessarily includes a turn-on duration in which the LEDs are turned on, so that it is possible to reliably record a state of turning on the traffic signal.
Furthermore, according to an imaging device of Patent Literature 3, the imaging period with which the image sensor captures images is out of synchronization with the flashing period of the LEDs of the traffic light machine, so that the LEDs of the traffic light machine that are turned on are reliably imaged within a certain period of time.